fix #26035

The tutorial of basic image operations https://docs.opencv.org/4.10.0/d3/df2/tutorial_py_basic_ops.html is still recommending using itemset. However this is now a deprecated functionality in Numpy 2.+ and in 1.+ there is a deprecation warning (or at least that is what they say).

We can still keep the warning about single pixel accessing as in the big picture that is still valid to not encourage people to access single pixels like this, but .itemset ain't a good option anymore.

Pull Request Readiness Checklist

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• N/A There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• N/A The feature is well documented and sample code can be built with the project CMake